The invention relates to a focussing arrangement, in particular for laser devices, comprising a focussing mirror arranged on a carrier which is displaceable in one plane, the focussing mirror having a spherical reflecting surface.
Such focussing arrangements frequently are used in laser devices, a precise adjustment of the respective focussing mirror being necessary for an exact focus adjustment. In conventional focussing arrangements, displacement tables (translation or measuring tables) comprising slide or roller bearings are used as carriers for the focussing mirror; the expenditures for these constructions are extremely high, and a bearing clearance which is always present causes a reduced rigidity of the element.
Thus, it is an object of the invention to provide a focussing arrangement of the initially defined type, which derives benefit from a simple and stable structure and which can be produced in a simple manner.
The inventive focussing arrangement of the initially defined type is characterized in that the displaceable carrier is formed by a rocking member which is at least approximately Z-shaped when seen in top view, the rocking member having a rocking longitudinal arm which is articulately connected on both ends with transverse arms, the longitudinal arm carrying the focussing mirror, in that the transverse arms are arranged so as to be pivotable about pivot axes perpendicular to the plane of displacement, and in that the center of curvature of the spherical reflecting surface of the focussing mirror lies at least substantially on an axis extending through the geometric center of the longitudinal arm, perpendicular to the plane of displacement.
In the present focussing device, thus, a Z-shaped rocking member is provided which is dimensioned and arranged such thatxe2x80x94when seen in top viewxe2x80x94the center of the rocking longitudinal arm coincides with the center of the circle described by the spherical reflecting surface of the focussing mirror. In case of a translatory displacement of the Z-shaped rocking member, the latter is also rotated about the axis extending through its geometric center, resulting in a corresponding rotation of the focussing mirror. This rotation of the focussing mirror, however, occurs about its center of curvature, i.e. the focussing mirror is xe2x80x9crotated along its reflecting surfacexe2x80x9d, and thus, such rotation will not cause a deflection of the focussed beam. The focussed beam therefore continues to extend along a given straight line, and merely the focal point is displaced along this straight line. Preferably, the center of curvature of the reflecting surface of the focussing mirror lies on the median between the incident beam and the line along which the focus of the focussing mirror is shifted during the displacement.
The focussing arrangement according to the invention may advantageously be used with laser devices, in particular with laser resonators, e.g. with laser devices of the type described in WO 98/10494 A. In particular, the invention may also be used in astigmatism-compensating laser resonators, such as described in Herwig W. Kogelnik, Erich P. Ippen, Andrew Dienes, Charles Shank, xe2x80x9cAstigmatically Compensated Cavities for CW Dye Lasersxe2x80x9d, Journal of Quantum Electronics, QE-8, No. 3, March 1972, pp. 373-379; in such a laser resonator arrangement, the laser medium, i.e. a laser crystal, in particular a Ti:S (titanium-sapphire) laser crystal, is arranged in the form of a plane-parallel plate, under a Brewster angle relative to the laser beam, and the laser beam is focussed in the laser medium by aid of focussing mirrors. However, by focussing into the laser medium arranged under a Brewster angle, an astigmatism is introduced, and the latter is compensated by tilting of the focussing mirrors.
Basically, however, the present focussing arrangement can be used quite generally in any arrangements where a light beam, or laser beam, respectively, is to be focussed by aid of a concave mirror, and where the focal point must be precisely adjusted.
For the configuration of the carrier and its bearing, e.g. a mode of construction comprising a block (as longitudinal arm of the rocking member) and with flat steel springs (as xe2x80x9carticulatedxe2x80x9d transverse arms) would be conceivable. Also conceivable are embodiments according to which rigid transverse arms are articulately connected to the longitudinal arm, and are pivotably mounted at their ends facing away from the longitudinal arm. With a view to a simple production, it is advantageous if the ends of the transverse arms of the Z-shaped rocking member which face away from the longitudinal arm are connected to a base carrier via thin material regions in the manner of an integral hinge. In this case, the base carrier preferably has a plate-shaped configuration.
Likewise, it is suitable if the transverse arms of the Z-shaped rocking member are connected to the longitudinal arm via thin material regions in the manner of an integral hinge.
In the present focussing arrangement, thus, preferably it is referred to a technique, already known per se, using carriers mounted via elastically deformable bearing elements, as it has been described under the term xe2x80x9cFlexure Arrangementxe2x80x9d in a catalogue by Melles Griot, p. 368/369. In the known arrangements, the carriers are adjusted by elastically deforming their bearing parts, yet this results in a parallel offset undesirable for the present applications. To avoid such a parallel offset, a structure called xe2x80x9cCompound Flexure Arrangementxe2x80x9d has been suggested which, however, is very complicated. As compared to this arrangement comprising a flexible multiple mounting, the present Z-shaped rocking member is substantially simpler in its production and, moreover, also more stable in operation.
With a view to the mass production sought, it has furthermore proven suitable if the Z-shaped rocking member is worked from the plate-shaped base carrier by providing openings while leaving the thin material regions. Production may start from a thicker starting plate (having a thickness of approximately 20 mm, e.g.), in which the respective grooves for the openings are worked in, e.g. milled, over part of the thickness of the plate (e.g., approximately 15 mm). Subsequently, the residual thickness (e.g. approximately 5 mm) may be removed by grinding so that the plate forming the base carrier including the Z-shaped rocking member will remain, having a thickness of approximately 15 mm. For working in the grooves, or openings, respectively, of course also other procedures, such as spark erosion or water torch cutting, may be used.
The thin material regions or material webs forming the integral hinges preferably have a thickness of from approximately 0.3 mm to 0.5 mm, in particular approximately 0.4 mm. With such dimensions, the desired elastic movability of the Z-shaped rocking member is ensured without breaking of the thin material regions. Surprisingly, it has been shown that thicker material webs will break more easily than such thin material regions having a thickness in the order of a few tenths of millimeters.
With a view to the required elastic deformability, on the one hand, and the desired stability, on the other hand, it is also suitable if the plate-shaped base carrier and the Z-shaped rocking member are made of aluminum or of an AlMg alloy.
To provide more conveniently space for accommodating other components of the focussing arrangement, in particular that component which is to-be focussed, externally of the Z-shaped rocking member, it is also If suitable if the longitudinal arm of the Z-shaped rocking member has a cranked shape, seen in top view.
As has already been mentioned, the present invention is advantageously used in laser devices, and accordingly, a particularly advantageous application is given in that a laser medium, e.g. a laser crystal, is arranged in the focus of the focussing mirror, externally of the Z-shaped rocking member. In this instance, the Z-shaped rocking member may carry one of the focussing mirrors of a laser resonator.